1. Field of the Invention
The present invention relates to a method of selecting an operating frequency in a wireless communications system and related communications device, and more particularly to a method of selecting an operating frequency corresponding to a multimedia broadcast multicast service for a user equipment in a wireless communications system and related communications device.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. To enhance multimedia functions in the 3G mobile telecommunications system, a protocol specification developed by the 3rd Generation Partnership Project (3GPP) provides Multimedia Broadcast Multicast Service (MBMS). MBMS is a point-to-multipoint bearer service and is established on an existing network architecture of the Universal Mobile Telecommunications System (UMTS), utilizing Internet Protocol (IP) packets as a medium. Thus, MBMS allows a single source entity to transmit data to multiple user equipments (UEs) simultaneously.
According to the protocol specification developed by the 3GPP, MBMS offers two delivery modes: MBMS Broadcast delivery mode and MBMS Multicast delivery mode. When interested in a specific MBMS service, the UE needs to order the MBMS service from a Broadcast Multicast Service Center (BM-SC) by establishing a service agreement including specific service provision phases. These two delivery modes have different service provision phases. The service provision phase of MBMS Broadcast delivery mode includes Service announcement, Session Start, MBMS notification, Data Transfer and Session Stop. The service provision phase of MBMS Multicast delivery mode includes Subscription, Service announcement, Joining, Session Start, MBMS notification, Data Transfer, Session Stop and Leaving. For realizing customized services, Joining and Leaving sessions utilized in MBMS Multicast delivery mode enable the BM-SC to execute authorization and payment recording for the UEs at the Joining session. The service provision phases mentioned above are well known in the art, and will not be described in detail.
On the other hand, from the standpoint of radio resource control (RRC), all logical data communication exchange channels, be they for providing data transmission exchange to the UE or for providing RRC layer control signal transmission exchange, are defined in the context of a Radio Bearer (RB). In the UE end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels. In the network end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels. For example, a UMTS Terrestrial Radio Access Network (UTRAN) transmits MBMS configuration to the UE by establishing RBs, and the UE stops receiving MBMS service by releasing the RBs. Besides, according to the RRC protocol specification provided by the 3GPP, an RRC state of the UE can be a disconnection mode or an RRC connection mode. The disconnection mode includes Idle mode, and the RRC connection mode includes CELL_PCH, URA_PCH, CELL_FACH and CELL_DCH states.
When providing the MBMS service, the UTRAN may obtain the number of the UEs ordering the MBMS service via a counting procedure, and can adopt either one of the following two transmission modes: point-to-point (p-t-p) transmission and point-to-multipoint (p-t-m) transmission. The UTRAN utilizes the p-t-m transmission to transfer data if the Broadcast mode is selected. On the contrary, if the Multicast mode is selected, the UTRAN utilizes either the p-t-p transmission or the p-t-m transmission to transfer data according to the number of the UEs. In the p-t-p transmission, transmission between the UEs and the UTRAN utilizes a dedicated channel, and thereby the Node-B establishes one dedicated channel for one UE to transmit data and configuration. Compared with the p-t-p transmission, the p-t-m transmission allows a core network (CN) to provide the data and the configuration for multiple UEs in the same cell simultaneously.
In the protocol specification developed by the 3GPP, three logical channels are defined to transmit corresponding MBMS information: MBMS point-to-multipoint Control Channel (MCCH), MBMS point-to-multipoint Traffic Channel (MTCH), and MBMS point-to-multipoint Scheduling Channel (MSCH). MCCH is utilized to transmit a corresponding MBMS configuration, including MBMS Neighboring Cell Information, MBMS Radio Bearer Information, MBMS Service Information, and other information. MTCH is responsible for transferring service data to the UEs. MSCH is used for notifying the UEs with scheduling information of MBMS services. With MSCH, the UEs are allowed to execute reception only when data begins to be transferred, without needing to keep a receiving state on throughout the whole service process.
Generally speaking, the UTRAN includes two or more carrier frequencies for providing a variety of services for UEs, and the UEs are appropriately distributed on these carrier frequencies. On the other hand, for the UE end, when the MBMS service the UE is interested in, or the state of the joined MBMS service, changes, the UE can perform a frequency selection procedure for following service reception operations. According to the meeting document R2-063613 of the 3GPP (3GPP TSG-RAN WG2 Meeting #56), when the UE receives an information element (IE) “MBMS Preferred frequency information” or an IE “MBMS PL Service Restriction Information,” or when the preferred MBMS service changes, the UE shall perform an MBMS frequency selection procedure for transferring a current operating frequency to a preferred frequency corresponding with an MBMS service prioritized by upper layers, so as to execute reception of the concerned MBMS service. The IE “MBMS Preferred frequency information” is included in a message named “MBMS MODIFIED SERVICES INFORMATION”, and is utilized for notifying the UE of the preferred frequency layer corresponding to the concerned MBMS services. The IE “MBMS PL Service Restriction Information” can be sent to the UE through a Radio Bearer Control message, a CELL UPDATE CONFIRM message or an MBMS GENERAL INFORMATION message, and is utilized for notifying the UE that the UTRAN will not provide any new non-MBMS services on the preferred frequency layer, which means that the preferred frequency layer is currently congested. In other words, if the IE “MBMS PL Service Restriction Information” is not included in the MBMS GENERAL INFORMATION message for the preferred frequency layer, there are still free radio resources available on the preferred frequency layer where the UTRAN can also provide the non-MBMS services.
Additionally, according to the protocol specification developed by the 3GPP, the above Radio Bearer Control message and the CELL UPDATE CONFIRM message are sent on a dedicated control channel (DCCH) for a dedicated UE, and the MBMS GENERAL INFORMATION message is sent on MCCH, which is broadcasted for all UEs in a cell. The Radio Bearer Control message can be a RADIO BEARER RECONFIGURATION message, a RADIO BEARER SETUP message, a RADIO BEARER RELEASE message, a TRANSPORT CHANNEL RECONFIGURATION message or a PHYSICAL CHANNEL RECONFIGURATION message, which are well known by those skilled in the art, and thus not narrated herein.
As the UE operates in different RRC states, the behavior of performing the MBMS frequency selection procedure is also different, and the following is aimed at a situation in which the UE operates in the CELL_PCH state, the URA_PCH state and the CELL_FACH state. Initially, if there exists one or more preferred frequencies for services included in a variable MBMS_ACTIVATED_SERVICES, which is stored in the UE, and the latest received CELL UPDATE CONFIRM message or Radio Bearer Control message did not include the IE “MBMS PL Service Restriction Information”, which is sent by the UTRAN on DCCH, the UE can proceed to following frequency selection.
Then, the UE determines a radio bearer state established with the UTRAN through a variable “ESTABLISHED_RABS” stored in the UE as a reference for performing frequency selection. For example, if the variable “ESTABLISHED_RABS” is not empty, the UE can keep staying on a current operating frequency when the current operating frequency of the UE is equal to a preferred frequency corresponding with an MBMS service prioritized by upper layers. On the other hand, when the current operating frequency of the UE is not equal to a preferred frequency corresponding with the MBMS service prioritized by upper layers, the UE can select the preferred frequency as the operating frequency, for which the IE “MBMS PL Service Restriction Information” is not included in the MBMS GENERAL INFORMATION message sent by the UTRAN. Otherwise, the UE shall select another preferred frequency corresponding to an MBMS service with lower priorities, for which the IE “MBMS PL Service Restriction Information” cannot be included in the MBMS GENERAL INFORMATION message.
The variable ESTABLISHED_RABS is used to store information about both established radio access bearers and signaling radio bearers in the UE, and thus a situation in which the variable “ESTABLISHED_RABS” is not empty means that the UE has established radio bearers, which may include the radio access bearers for transmitting packet data or the signaling radio bearers for signaling only. In this case, when the current operating frequency of the UE is not equal to a preferred frequency corresponding to the MBMS service prioritized by upper layers for which the IE “MBMS PL Service Restriction Information” has been received in the MBMS GENERAL INFORMATION message, the UE cannot select the preferred frequency as an operating frequency. That means, in the situation, the UE cannot select the frequency, which is determined to be congested by the UTRAN, for receiving the concerned MBMS service.
As well known by those skilled in the art, the signaling radio bearer does not consume too much radio resource. However, according to the above-mentioned meeting document, even if the established radio bearers of the UE only include the signaling radio bearers, the UE still cannot transfer to the preferred frequency, which is determined to be congested by the UTRAN, so that the UE will lose the opportunity for receiving the concerned MBMS service.